This invention relates to sheet transfer apparatus having a plurality of generally horizontally spaced grippers for gripping the top surface of the sheet and transporting it to a work station where it is maneuvered into a correctly aligned position for a sheet forming operation such as edge folding. Prior art machinery for accomplishing these objectives has been of a complex, massive, and expensive design which has not been as maneuverable.
Because of their size and thickness, blanks of the type used for forming refrigerator doors, for example, are somewhat flexible and must be handled with considerable care. It is necessary that the suction cups which grip the sheet maintain a sealed engagement with the top surface of the sheet throughout the transfer operation in order to prevent them from dropping and being damaged. For this reason, it is impractical to move the suction cups relative to the sheet once they have been engaged with the sheet and a vacuum has been applied to the cups. It is, further, necessary in automation operations, wherein the workpieces are moved from one station to another along a manufacturing line, that the refrigeration doors or the like, be manufactured with great precision, and the emphasis today is on greater and greater speeds of production. A further emphasis today in such machinery, is the versatility to handle different sizes so that the production lines can be quickly changed over to produce doors of different size, with a minimum of changeover time involved. Prior art machines have not been sufficiently versatile in this respect to the extent necessary to be as useful in today's high production rate automated production lines. The prior art machinery known to applicant employed a transfer carriage with a large plate having a plurality of suction cup grippers fixed thereto such that the grippers were not shiftable on the plate relative to one another or the plate. The relatively massive plate was itself shiftable on the carriage when compliance was required, and this shifting was opposed by large return springs. This prior art machinery was a high friction, inertia oriented system which was nowhere near as maneuverable or compliant as the compliance apparatus of the present invention where individual grippers can shift relative to and responsive to the shifting of other grippers.